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    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Look North in February 2013 – Watch the Great Bear Come out of his Cave!

When you look to the northeast early on a February evening do you see this:

or maybe this:

Used by permission from the Linda Hall Library of Science, Engineering & Technology.

or perhaps this?

It all depends, of course, on your imagination, but for me I see something like the last image. Even that doesn’t quite capture what my imagination wants to do with these stars. What I see is a huge and rather grumpy bear, emerging from his cave a bit early after hibernating through a few rough months, and now he’s stretching – and clawing – his way up my sky, and he is magnificent!

But I admit, for years it wasn’t that way. I saw instead what I suspect many people see – the Big Dipper rising. And I knew, sort of vaguely, that this asterism – one of the most familiar in the world – was a major portion of the constellation of the Great Bear, Ursa Major.  But really, large as the Dipper is, it’s just the hind quarter of the Big Bear, which is really large, and when I finally took the time to trace out his head and ears and front and rear paws, he quickly became one of my favorite constellations – one of the rare ones like Orion and Scorpius that really look like what they depict.  And funny – I can’t explain why, but I seldom see it as a bear except at this time of year when it is rising. Then it seems to dominate my northern sky and my imagination.

Oh – did I say it looks like a bear? No – I should have said it looks like a bear no one has seen except in the sky – a bear with a long tail! I don’t know why that is. I assume it is because of the second depiction, which is how Johann Bayer pictured the Great Bear in his “Uranometria,” a breakthrough star atlas published in 1603.  Bayer was a lawyer, not a hunter. Maybe he had never seen a bear?

The first depiction, a Stellarium screenshot, is the best one to use as a guide for finding the correct stars. Besides the Dipper stars, there are a dozen more that trace out his main features, and all of these are either third magnitude, or on the brighter side of fourth magnitude – that is between 3.5 and 4, so they should be visible from most locations – assuming, of course, you are in mid-northern latitudes.  The chart that follows gives a view of the Bear in context with the rest of the northern sky in February.

About one hour after sunset, look north and you should see a sky similar to the one shown in our chart below. The height of Polaris, the North Star, will be the same as your latitude. Polaris stays put.  Everything else appears to rotate about it, so our view of all else changes in the course of the evening – and from night to night. It’s a good idea to check the north sky every time you observe to get a sense of how things are changing and to orient yourself.  Notice that the “W” now looks more like an “M” as it starts to roll on down into the northwest.

Click image for larger view. (Chart derived from Starry Nights Pro screen shot.)
Click here to download a printer-friendly image of the above chart.

Look east in November 2012 for the “Eye of Sauron” star and its “zombie” planet!

November brings us our southernmost – and northernmost – guidepost stars, Fomalhaut and Capella. And  fresh off the press of NASA for Halloween – a zombie planet! Now you see it – now you don’t – now you do!  But first, the normal.

The positions of Capella and Fomalhaut in the sky mean that for Northern Hemisphere observers Fomalhaut is the guidepost star we see for the shortest amount of time – and Capella is the one we see the longest.

In fact, for many, Capella is visible during some hour every night of the year – and for those north of latitude 45 degrees, it is circumpolar – that is, it never sets. But lonely – and freshly fascinating – Fomalhaut just puts in a relatively brief appearance low to the south.

From NASA:”This image, taken with the Advanced Camera for Surveys aboard NASA’s Hubble Space Telescope, shows the newly discovered planet, Fomalhaut b, orbiting its parent star, Fomalhaut. The small white box at lower right pinpoints the planet’s location. Fomalhaut b has carved a path along the inner edge of a vast, dusty debris ring encircling Fomalhaut that is 21.5 billion miles across. Fomalhaut b lies 1.8 billion miles inside the ring’s inner edge and orbits 10.7 billion miles from its star.” Click image for larger version.

Fomalhaut is “lonely” because there are few bright stars in its vicinity. It is “freshly fascinating” because early in this century the Hubble Space Telescope got a fantastic picture of a disc of “debris” surrounding it, showing this young star to be in the throes of forming planets. Then in 2008 scientists announced they had actually found a planet circling Fomalhaut (see photo above), the first planet outside our Solar System to be seen with visible light. Cool! Add to this the fact that the Hubble photograph of Fomalhaut could be easily mistaken for the Eye of Sauron, and for fans of the Lord of the Rings movie triology, Fomalhaut becomes especially memorable. (For more on the “Eye of Sauron” go here.)

But wait, this just in!

Vital stats for Fomalhaut (FO-mal-ought)

• Brilliance: Magnitude 1.16; its luminosity is the equal of 16 Suns.
• Distance: 25 light years
• Spectral Types: A3V
• Position: 22:57:39, -29:37:20°

After reading this description, click on the chart for a larger version. About an hour after sunset, November evenings offer us an eastern sky filled with a host of asterisms both large and small. A good starting point for the naked eye is the Great Square of Pegasus. From one corner of it you can find Andromeda’s Couch which ties into what I call the “Demon’s Triangle” because it is anchored by the eclipsing variable, Algol – the “Demon Star.” The “W” of Cassiopeia should be obvious – and there are three asterisms shown that are best seen with binoculars. The “Hockey Stick” and “Water Jug” should fit in a low power binocular field, while only half of the “Circlet” will fit. Capella anchors our chart to the north, with Fomalhaut to the south. I included Deneb Kaitos because while it is a little dimmer than Fomalhaut, it could be mistaken for it. Wait an hour or so and you’ll see brilliant Jupiter rise in the east to dominate this portion of the sky in 1012. (Prepared from Starry Nights Pro screenshot.)

Click here to download a printer friendly version of the above chart.

Finding Fomalhaut

As always, it’s easiest if you start looking in the east 45 minutes to an hour after sunset when in the twilight only the brightest stars are visible as shown on our chart. Fomalhaut is the brightest star south of southeast and about a fist and a half above the horizon 45 minutes after sunset. I emphasize “star” because a bit later int he evening in 2012 Jupiter is in the  east as well, but significantly brighter. Trailing well behind Fomalhaut – to the east – and lower still is a second magnitude star (same brightness as the North Star) called Deneb Kaitos. Don’t mistake it for Fomalhaut.

If you have learned the Great Square – see this post – then the two stars that form the western edge of that square can be used, as pointer stars. Drawing an arrow through those two stars leads you to Fomalhaut. You could also wait until a couple of hours after sunset when you would find Fomalhaut very close to due south. Even then, from my latitude of 41.5° N it is not quite two fists (19°) above the southern horizon.

Ahhh Capella!

Capella is distinctive because it’s not “a” star – it’s two! But these two, bright, yellow suns are so close together that you’ll always see them as one, even if you use a large telescope. Together they make a star that rivals Vega and Altair, now well into our western sky, in brightness. (See Summer Triangle chart here.) In fact Capella is the third brightest star in the Northern Hemisphere – but that’s a tad deceptive because it doesn’t count Sirius – the brightest star that most Northern Hemisphere observers can see, although technically Sirius is in the Southern Celestial Hemisphere, since it is below the celestial equator. But you don’t have to worry about such technicalities to enjoy a view of Capella. Just look near the horizon to the northeast. You will need a very clear horizon, however, especially at the start of the month because then Capella will not even be one fist above the horizon.

Just as Fomalhaut is a bit south of southeast, Capella is a bit north of northeast.

It’s easiest to find Capella if you start 45 minutes to an hour after sunset. Choose a spot with a clear horizon to the northeast and watch for a bright star to appear very near the horizon. Like all bright stars near the horizon, Capella will twinkle and flash in different colors because you are seeing it through a lot of atmosphere. It won’t show its soft, golden hue until it is much higher in the sky. Even a veteran skywatcher can be fooled by this. Recently my wife was looking to the northeast on a fall evening and saw what she thought was Capella. But it was so bright and blinking red and green so distinctly, that she changed her mind and decided it was an airplane! (There’s an airport off in that general direction.) When after a minute or so it hadn’t moved, she knew her first thought was correct – but boy it made a convincing airplane!

For me, Capella marks a graceful arc of bright stars and asterisms that circle the north celestial pole. If you have been following these directions for a few months, look at Capella, the “Bow” of Perseus, and the “W” of Cassiopeia to see what I mean. Watching these move in the course of a single night – or from month to month – always gives me a real sense of how, from our vantage point, all the stars appear to circle Polaris.

As mentioned, Capella is really a complex multiple star. Its two main components are both yellow giants dubbed Aa and Ab, but there are two more stars in this family. However, they are a pair of red dwarfs only visible in a telescope and are so far away from the two bright stars that they take more than 1,000 years to complete an orbit. The two bright stars orbit in just 104 days. James B. Kaler, in his book The Hundred Greatest Stars, says this about the Capella twins:

These two magnificent giants are separated by about the distance between Venus and the Sun. A resident on a ‘Jupiter’ ten times further out would see two ‘Suns’ about half a degree across (similar to the Sun in our own sky), separated at maximum by some 6 degrees, one setting right behind the other.
So when you find Capella, pause – picture yourself on the Jupiter-like planet with these twin yellow Suns in your sky!

Vital stats for Capella (kah-PEL-ah)

• Brilliance: Magnitude .08; its luminosity is the equal of 16 Suns.
• Distance: 42 light years
• Spectral Types: G8/G0
• Position: 05:16:41, +45:59:53

In this month’s chart I identify three relatively dim asterisms as good objects for your binoculars – there’s also the magnificent Andromeda Galaxy barely visible to the naked eye if you have very dark skies, but certainly a small blurry patch in binoculars. The arrows on the chart show two paths to tracking it down by star hopping. Found it? Pat yourself on the back. You are looking at about 300 billion stars and you are looking back in time about 2.5 million years!

The “Water Jug” of Aquarius is a nice binocular object. To me it looks just like a three-bladed airplane propeller.  The “Circlet” is part of Pisces and while quite faint, is easy to trace out in binoculars, though you will have to scan about a bit to see it all. It doesn’t fit in a single field of view – at least in most binoculars.

What I dub the “Hockey Stick” are the three brightest stars of Aries, the Ram. The faintest of these is an easy and beautiful double – a nearly perfectly matched pair if you have small telescope, point it at them and enjoy.

Still with us!

Other bright guide stars and asterisms introduced in previous months that are still readily seen include the Summer Triangle of Altair, Deneb, and Vega, which is high over head and crossing into the western sky. Arcturus is just above the horizon in the west, the Big Dipper just west of north and hugging the horizon, and the Teapot is diving into the ground in the southwest. And, of course, we have the “Bow” of Perseus with “Algol” the “Demon” star, the “W” of Cassiopeia, the “home plate” of Cepheus, Andromeda’s Couch, and the Great Square.

Venus transit June 5/6, 2012 – you don’t want to miss this one!

UPDATE – The transit went well for many folks throughout the world. For a personal observing report and a few pics I took, go here. For lots and lots of pictures , go here.  And for other transit observing experiences, go here.

 

Transit as seen from Westport, MA through a hole in the clouds.

Study this NASA map to see whether you are slated to see all of the transit of Venus on June 5/6, 2012, or part near the time of local sunrise, or part near local sunset. (Click image for larger version.)

On June 5/6, 2012, most of the world will have the chance to see all – or part – of a once-in-a-lifetime  event – a transit of Venus across the face of the Sun.  CAUTION: To view this, even with the naked eye, you must use proper protective filters. Binoculars and telescopes must be equipped with such filters and if not, used only to safely project an image of the event – not looked through.

That said, here are three shots simulating the event as seen from Westport, MA. From this East Coast location we will see only the first couple of hours of the transit, then our view will be interrupted by sunset. Notice that Venus will appear to enter near the “top” of the Sun, This location and path vary with your position on Earth. (At the end of this post are several links. The second of these links gives you specific information on the time and the path of Venus across the Sun as seen from your location. In the images below, the Sun is festooned with sunspots and other features. Such features may or may not be seen depending on what is happening on the Sun at the time of the transit and on the type of solar filter used to view the event.

Predicted path of Venus transit across the face of the sun as seen in astronomical telescope (flips image horizontally) from Westport, Ma. Click image for larger view. (Prepared from Starry Nights Pro screen shots.)

Did you find the images exciting? Probably not. But it should give you some idea of what to look for on June 5, and there is no substitute for seeing the real thing as it happens.  There’s also no substitute for understanding what it is you’re viewing and why – besides the fact that there won’t be another such transit for more than a century. No wonder so many people are very excited about seeing it. I’ve already seen one such transit – as have many others – for these events come in pairs fairly close together, and the last one was visible just eight years ago.  But I still will make every effort to see this one, and if the weather forecast says my local view is likely to be obscured by clouds, I’m ready to drive a couple of hundred miles to get to some place that’s clear.

Here, in a nutshell, is why I find this event so exciting:

  • There won’t be another chance to see a transit of  Venus until 2117.
  • On display will be the full majesty  – and magic – of our gravitationally-powered solar system where Venus – a body almost as large as the Earth – passes directly between us and the Sun at a distance of bout 30 million miles.
  • More than 200 years ago many scientists risked life and limb to travel to distant locations on the Earth in attempts to view the transit and accurately time it.  Such observations, they hoped, would unlock the secrets of the size of our solar system – secrets that despite their best efforts remained hidden. As noted in the New York Times, “Sea travel was so risky in 1761 that observers took separate ships to the same destination to increase the chances some of them would make it alive.”
This event is covered in so many different ways with wonderful graphics, wonderfully accessible details about when you can see it from your location, and terrific stories, that I would be trying to reinvent the wheel to repeat it all here.  Instead, I urge you to take advantage of the links below.

Some useful transit links

Events May 2012: Ring of Fire in the West, the fattest Moon, thinnest Venus, and parade of twins

I would love to see the May “ring of fire” – an annular solar eclipse, but it’s too far away for me. However, if you live anywhere near the eclipse path which starts in Asia and ends in the western United States, May 21/20 could prove special. Sky and Telescope says that while the eclipse weather for Asia tends to be bad at this time of year, the weather tends to be very good in the Western United States. And I have to admit, one of the little fascinations of this event for me is it starts on May 21 and ends on May 20 – yep, time can run backwards ;-).

Of course, if you’re not in the eclipse path,  May offers some other choice viewing for the unaided eye and binoculars :

But first, a few more eclipse notes

Path for the May annular solar eclipse. Click for larger image and for many more detailed eclipse maps, see the links a couple paragraphs down.

OK, it’s not time that’s running backwards – it’s the shadow of the Moon across the Earth and the shadow starts in Asia on May 21, eventually crosses the International Date Line, and then ends in Texas on May 20.  And – just to be clear – an annular eclipse  is not the same as a total solar eclipse, nor as a partial eclipse.

The annular eclipse is better than the typical partial eclipse – which is still fascinating – but it is not the stunner that a total solar eclipse is. It is “annular” – the word means “ring shaped” –  because the Moon is so distant from the Earth at the time of the eclipse that it is not quite large enough to totally cover the Sun and so there will be a ring of light – thus “ring of fire” – at “totality” which is probably better thought of as “mid-eclipse” since it won’t be total.  The Moon will cover  94% of the Sun’s diameter, but that remain 6 percent will still generate a lot of light. It will be noticeably darker at any given location during those few minutes most of the Sun is covered, but it will not be nearly as dark as when there is  a total  eclipse.

For a full selection of detail eclipse maps and other information for different sections of the world, please go here. And for far more detail on everything to do with the eclipse, go here.

It’s a BIG – I mean REALLY BIG – full moon!

As noted, the annular solar eclipse occurs because the New Moon in May is so far from the Earth and thus appears so small that it’s disk does not cover the Sun. On May 5, when the Moon is full, it is closest to us in it’s orbit – as close as it gets at the time of full Moon in 2012 and thus gives us an especially large full Moon.

How large is large? Well, when it rises on May 20th on the East Coast  of the U.S. it will be right near it’s minimum distance of 221,457 miles and will show a disc of roughly maximum size – about 33′ 30″ in diameter.

And on May 20, when it is creating the  annular eclipse of the Sun, it will be very close to it’s maximum distance from us of 252,712 miles and it’s disc will be roughly 29’24” in diameter. (Of course it will be too close to the Sun for us to see that night, but in the next few days the crescent will emerge and that crescent will  include a lunar disc shining by the reflection of light from the Earth and  an especially small one at that.)

Why the “roughly” and “abouts” for sizes and distance in those sentences? Because the Moon is constantly in motion and constantly changing size and distance from us. So while there’s a correct size and distance for a specific instant – such as Moon rise at my exact longitude – we have to be more general when we’re using numbers that cover a date and time for Moon rise over different parts  of the Earth, or an extended event like the annular eclipse.

So will you be able to tell that it’s big? I mean, if you do the math you will  see  that we’re talking of a change from largest to smallest of roughly  four minutes of arc.  Can we detect such a change? Yes. Look at the images below. See a difference?

The moon when closest – and farthest – from us. To simulate the experience of two Moon’s at these different distances from us, click on the image, print the resulting picture, and tape it to the wall 12 feet away from you.  (Images from Fourmilab, by John Walker
– public domain)

Unfortunately you never get to see such a comparison live, in the sky. In fact, you will have to wait until  Nov 28, 2012  to see the smallest full Moon this year.  At that time it will be right out there near it’s maximum distance from us and show a minimum disk size. (Hmmm… would be fun to photograph the May 5 Moon rise near a certain landmark, then do a similar photo at the same spot  on November 28. nut. of course, it would have to be a portable landmark you move into place because the two will not rise in the same location – in fact, they will be quite far apart.)

Now, if you want to crunch the numbers, consider four minutes of arc – that is frequently quoted as the distance we can detect with our naked eye. So, for example, two stars that are four minutes of arc apart and the same brightness we could split without optical aid. So why is it obvious the Moon is bigger when it’s a difference of just four minutes?  Remember that  with the Moon we’re citing a diameter, but what we see is an area. The area turns out to be 16.75 squared time 3.14 = 881  vs  14.7 square C 3.14=679 – a factor of 202 – nearly one fourth!  So if you calculate the area of the Moon’s disk visible to us when nearest and when farthest away the difference is significant!

There’s one caution, though.  The Moon and Sun when near the horizon always – ALWAYS – look much larger than they do when high in the sky. This has nothing to do with their  actual distance from us, or size. Take a picture of that Moon near the horizon and the picture will show a Moon that looks much smaller than you remember seeing.  The reason is what’s commonly known as the Moon Illusion – and that a whole different story. For a formal discussion of a complex topic, take a look here.

Complex as the Moon Illusion is, when you begin to understand the constantly shifting position of the Moon – develop a gut feeling by watching the changes – you really begin to appreciate the incredible complexity of landing a space mission there. And those with long memories will recall that  landing on the Moon is hardly a slam dunk.

In 1959 they [the Soviet Union] launched Luna 1, which missed the Moon by 3,728 miles (5,998 km). They followed that flight with a spectacular circumlunar orbit by Luna 3, which gave us our first pictures of the far side of the Moon.

 The development of probes in the United States also revolved around the Moon at this time. After several unsuccessful attempts to reach the Moon with the Pioneer series, the National Aeronautics and Space Administration (NASA) launched the Ranger series. It planned to crash-land the spacecraft onto the Moon’s surface, taking photos up until impact. The first few probes were unsuccessful, but the last three– Ranger 7, Ranger 8, and Ranger 9–took over 17,000 pictures beginning in 1963.   source

So this whole business of the lunar orbit around us is complex and is really better thought of as the Moon’s orbit weaving inside and outside our own in the course of each month  as we both travel around the Sun.  So I hope the weather cooperates and you get to bask in May’s full Moon and contemplate our deceptively simple relationship to our companion planet. (Yeah – that’s another thing – many regard the Earth and Moon more as a double planet system – the moons of other planets are much smaller in relation to their planet than our Moon is in relation to us.)

And now that we have the Moon on stage, how about that svelte Venus?

Venus goes through phases like the Moon as well. But what’s interesting about the Venus phases is that it is “full” when it’s farthest from us – and it’s a thin crescent when it’s closest to us. That really changes the dynamic. With the Moon there’s no such relationship. It can be a crescent and close, or a crescent and far away.

That thin crescent in May 2012  grows to more than 56″ in diameter by the end of the month.  Yes, those are seconds or arc. It’s still much smaller than the Moon where we measure it’s angular size in minutes. Remember, one minute equals  60 seconds, so the full Moon near the beginning of May is about 35 times as large as Venus is to our eyes near the end of May.  Can we see something this small as a crescent? I think it would be very difficult with the naked eye, but handheld binoculars will magnify it  7-10 times – that makes its crescent form identifiable.

BUT IF YOU PUT THIS TO THE TEST, PLEASE BE CAREFUL. VENUS RAPIDLY APPROACHES THE SUN THIS MONTH. So I suggest if you try to see it in daylight, you do so in the early part of the month. It is a crescent on May 1, though at 44″ a smaller one, it is still large enough to be detectable. JUST AVOID LOOKING AT THE NEARBY SUN WITH YOUR NAKED EYE AND/OR BINOCULARS OR A TELESCOPE.  For more details on how to safely see Venus in Daylight  go here.  On May 1, 2012 Venus is still about 36 degrees from the Sun.  By May 10, 2012  it’s about 30 degrees away and by the 20th it’s 20 degrees away. That is really getting too close for comfort as far as I’m concerned. In the second half of the month I would only look for Venus after sunset – even when taking the precaution of putting a building between me and the Sun.  I value my eyes far too much to play games.

But the point is. we have some interesting dynamics at work here in terms of its brightness. You would think Venus would be brightest when it was “full” or near full – just like our Moon. But it isn’t. And you might think it would be brightest when it is closest to us – but then it’s just a thin crescent that we see, so it isn’t.  Actually, there’s a compromise position about one third of the way through May 2012 when it is both a crescent – less than 20% of the disc illuminated – and near it’s brightest at magnitude -4.7. After that it gets to be a larger crescent, but it also dims some because so little of the disk is lit. Still, even at the end of the month with just one percent illuminated it is shining at a dazzling  – 4.1.

But I  hasten to add that in that last week Venus will be more and more difficult to see. Fifteen minutes after Sunset it is just five degrees above the horizon ( at my mid-Northern latitude) and in the bright twilight would require a clear and unobstructed western horizon to see.

This plunge towards the Sun, is, of course, heading for that twice-in-a-century event, the Venus transit of the Sun.  We already had a shot in 2004 at a transit of Venus, but these events come in pairs. The June 5, 2012 transit is the second of the pair for this century.  I’m really looking forward to this one. For North America, only the first part of the transit will be visible with sunset interrupting it. Weather prospects are pretty problematic too.  I plan to set up special equipment, properly filtered for safely viewing the Sun, in my favorite location with an unobstructed western horizon. But I’m also scouting other locations within reasonable driving distance, if the weather looks more favorable  north or west of here. I’ll publish a separate post with more details for viewing the transit which will be widely available from different locations on Earth and provides a way to relive some wonderful scientific history. In the 18th and 19th centuries viewing a transit of Venus was regarded as the key that would open the door to being able to calculate the actual size of our solar system. That provided the impetus for some fascinating – and downright heroic – scientific expeditions around the world.

May’s Parade of Twins – Saturn/Spica, Mars/Regulus and the Real McCoy!

The “Heavenly Twins,” Castor and Polux are still with us in May, high in the West an hour or two after Sunset. But they are joined by two other closer pairs of bright “stars” that have a special fascination do to color contrast and motion. High in the southeast are  Saturn and Spica.  And high overhead and favoring the southwest are a third pair, good as such only for the start of the month, Mars and Regulus.

Click image for larger version of this chart – prepared from Starry Nights Pro screen shot.

The pairs present some really nice color contrast , something that will be more apparent if you look at them in binoculars. Saturn has a yellowish tint, while it’s companion, Spica, is an icy blue. Mars is orange-to-red, while Regulus is white with just a hint of blue. Castor is white, but Pollux has a yellow tint.  (For more on the color of stars, please go here.)

Quite a line-up, really, since all are very bright “stars.”  Castor, while bright, is the dimmest of the six.  It is magnitude 1.56 and the convention is to say that first magnitude  runs from magnitude .5 to magnitude 1.5.  So Castor  misses first magnitude  by hair where the others are either first magnitude or zero!

Pollux is magnitude 1.15.  Saturn starts the month at magnitude .5, then joins the zero magnitude class by climbing to magnitude .3  by the end of the month.  And Mars? Mars is the most fickle  of the group. It starts off the month a perfect 0 magnitude, but by the end has dimmed to .5, so it’s headed for the first magnitude class. It also breaks the twin pattern – that is, at the start of the month it is is less that 6 degrees from Regulus (magnitude 1.34), but it more than doubles that distance by the end of the month.  Saturn barely changes it’s relationship with Spica (magnitude .96), being about 4°50′  distance all month – and, of course, Castor and Pollux are, for all practical purposes, constant at a separation of 4°30′.

Look East! Slide down to Saturn and Spica in May 2012!

It’s a tad easier to find Saturn and Spica if you found Arcturus in April, but if not you’ll simply get a “two-for-one-special” for your effort this month. As always, start about 45 minutes to an hour after sunset. In May 2012 there should be four bright “stars” in the East, but one is a planet. In order from north-to-south they are Vega, Arcturus, Saturn, and Spica. As the sky gets darker the bright stars of the Big Dipper, high in the northeast, will guide you.

All you really want is the three stars of the Dipper’s handle. It forms a wonderful arc, and if you follow the curve of that arc, it will always take you to Arcturus. Continue the same curve for about the same distance, and you will come to the beautiful – but fainter – blue-white gem, SpicaSaturn is very close to Spica, though yellowish, compared to the rich lue of the star. And Vega is way at the other end – just coming up in the northeast. It is very close to the same brightness as Arcturus. All of which, I’m sure, is much easier to grasp if you simply look at this month’s “look east” chart.

Notice that the distance between the last star in the handle of the Big Dipper and Arcturus is almost exactly the same as the distance between Arcturus and Spica - a good way to make sure you're looking at the right star! Also note the movement of Saturn from 2011 to 2013. Click image for larger version. (Developed from Starry Nights Pro screen shot.)

Click here to download a printer-friendly version of this chart.

We dealt with Arcturus last month. Saturn will be in our sky most of the night and as always is a treat for the small telescope user. From a naked eye perspective,  it’s fun to remember that the name “planet” means “wanderer” in Greek, but all “wanderers” are not created equal. Mars, Venus, and Mercury move  so quickly in our night sky that you can easily mark their changes over a period of a few days -certainly a week.  Saturn is much more sluggish.

Look at the chart and you’ll see how little Saturn changes position over the course of an entire year – it moves roughly 12 degrees.  To see this,f ind Saturn. Hold your fist at arms length so Saturn is just below it. Just above your fist is where Saturn was last year. Put Staurn on top of your fist and just below your fist is where it will be next year. So how long will it take Saturn to get around the sky to roughly the same position? Well, 360/12 = about 30 years!  Now if you think a moment, the Moon takes about 30 days to get around our sky – and that means the Moon moves each day about 12  degrees –  the same distance covered by Saturn each year.  All of which should tell you that it would be reasonable to assume Saturn is much farther away from us than the Moon – which, of course, it is.

None of this is rocket science or in any way  profound, but I find it interesting to contemplate as I look up and see Saturn. I measure that distance it will travel in the next year and in my mind’s eye I perch above the Solar System and I see a long thin pie slice reaching from me to Saturn’s distance orbit and this helps me keep things in perspective – gives me a better intuitive feel for the neighborhood in which we live.  OK – for the record Saturn is moving at about 22,000 miles an hour, Mars about 54,000 miles an hour in a much shorter orbit, and we’re whipping right along close to 67,000 miles an hour – and we don’t even feel the wind in our face! Oh – and Saturn’s actual orbital period is 29.458 years.

On to this month’s new guidepost stars!

Vega and Spica are each fascinating stars, but let’s start with Vega. Shining brightly not far above the northeastern horizon as the evening begins, Vega comes about as close to defining the word “star” as you can get. In “The Hundred Greatest Stars” James Kaler calls it “the ultimate standard star” because its magnitude is about as close to zero as you can get (.03) and its color is about as close to white as you can get. (If you’re one of those who assumed all stars are white, you’re forgiven. Individuals vary in their ability to see different colors in stars and for everyone the color differences are subtle – in fact I think of them as tints rather than colors. )

It’s hard not to be attracted to Vega when you read Leslie Peltier’s wonderful autobiography, “Starlight Nights.” Vega was central to his astronomical observing throughout his career because he began with it when he first started reading the book from which I got the idea for this web site, “The Friendly Stars” by Martha Evans Martin. Peltier wrote:

According to the descriptive text Vega, at that very hour in the month of May, would be rising in the northeastern sky. I took the open book outside, walked around to the east side of the house, glanced once more at the diagram by the light that came through the east window of the kitchen, looked up towards the northeast and there, just above the plum tree blooming by the well, was Vega. And there she had been all the springtimes of my life, circling around the pole with her five attendant stars, fairly begging for attention, and I had never seen her.

Now I knew a star! It had been incredibly simple, and all the stars to follow were equally easy.

Vega went on to be the first target of the 2-inch telescope he bought with the $18 he made by raising and picking strawberries. (This was around 1915.) And Vega became the first target for every new telescope he owned until his death in 1980. If you still don’t know a star, go out and introduce yourself to Vega early on a May evening. Even without a plum tree to look over, you can’t miss her! And once you’ve done that you’re well on your way to making the night sky your own.  (And yes, Vega is the star from which the message comes in Cal Sagan’s book/movie “Contact.”)

Vital stats for Vega, also known as Alpha Lyrae:

• Brilliance: Magnitude .03 ; a standard among stars; total radiation is that of 54 Suns.
• Distance: 25 light years
• Spectral Type: A0 Dwarf
• Position: 18h:36m:56s, +38°:47′:01″

Spica, a really bright star – honest!

Spica is truly a very bright star, but the numbers you may read for its brightness can have you pulling your hair. That’s because there are at least four common ways to express the brightness of Spica and other stars, and writers don’t always tell you which way they’re using. So let’s look at these four ways and see what they mean for Spica.

The first is the most obvious. How bright does it look to you and me from our vantage point on Earth using our eyes alone? We then assign it a brightness using the magnitude system with the lower the number, the brighter star. (For full discussion of this system, see “How bright is that star?”)

By this measure Spica is 16th on the list of brightest stars and is about as close as you can come to being exactly magnitude 1. (Officially 1.04)Thopugh I should add here that the number really marks the midpoint of a magnitude designation – that is, any star that is in the range of magnitude .5 to magnitude 1.5 is called “magnitude 1” and so on for the other numbers ont he scale.

But that scale talks about what we see. It doesn’t account for distance. Obviously if you have two 60-watt light bulbs and one is shining 6 feet away from you and the other 1,000 feet away, they are not going to look the same brightness. But if we put them both at the same distance – say 100 feet – they would look the same. So it is with stars. To compare them we pretend they all were at the same distance – in this case 10 parsecs, which is about 32.6 light years. Put our Sun at that distance and it would be magnitude 4.83. (That’s about as faint as the fainest stars we see in the Little Dipper.) We call that its absolute magnitude.

The absolute magnitude for Spica is -3.55 – not quite as bright as dazzling Venus.

Wow! That’s pretty bright compared to our Sun! Yes it is. Sun 4.83; Spica -3.55. Don’t miss the “minus” sign in front of Spica’s number! That means there’s more than eight magnitudes difference between the Sun and Spica. And that relates to the next figure you are likely to see quoted. Something that is called its luminosity. Luminosity compares the brightness of a star to the brightness of our Sun. Unfortunately, the term is often misused – or poorly defined. Thus in the Wikipedia article I just read on Spica it said that “Spica has a luminosity about 2,300 times that of the Sun.” Yes, but what does that mean? It means that if we were to put the two side by side, Spica would appear to our eyes to be 2,300 times as bright as our Sun.

That is bright! But there’s more, much more. Spica is also a very hot star – in fact one of the brightest hot stars that we see with our naked eyes. But we miss most of that brightness because most of it is being radiated in forms of energy that our eyes don’t detect. In the case of Spica, that is largely ultraviolet energy. The Wikipedia article actually listed Spica’s luminosity twice, and the second time it gave it as “13,400/1,700.”

Oh boy – now we have Spica not 2,300 times as bright as the Sun, but more than 13,000 times as bright. Now that IS bright – but is it right? Yes! So why the difference? Again, the first “luminosity” given – 2,300 times that of the Sun – is measuring only what we can see with our eyes. The second is measuring total amount of electromagnetic radiation a star radiates and is properly called the “bolometric luminosity.” And why two numbers for that last figure? 13,400/1,700? Because while Spica looks like one star to us, it is really two stars that are very close together and one is much brighter than the other. So what we see as one star is really putting out energy in the neighborhood of 15,100 times as much as our Sun.

This can get confusing, so I suggest you remember three things about Spica.

1. It defines first magnitude, having a brightness as it appears to us of 0.98 – closer than any other star to magnitude 1.

2. It is really far brighter (magnitude -3.55), but appears dim because it is far away – about 250 light years by the most recent measurements.

3. It is very hot – appearing blue to our eyes – and because it is very hot it is actually radiating a lot more energy in wavelengths we don’t see, so it is far, far brighter than our Sun.

Spica is the brightest star in the constellation Virgo, one of those constellations where you can not really connect the dots and form a picture of a virgin unless you have an over abundance of imagination. Besides, the remaining stars are relatively faint. That’s why we focus on the bright stars and sometimes those simple patterns known as “asterisms” and use them as our guides.

Vital stats for Spica, also known as Alpha Virgo:

• Brilliance: Magnitude .98 ; as close to magnitude 1 as any star gets; a close double whose combined radiation is the equal of 15,100 Suns.
• Distance: 250 light years
• Spectral Types: B1,B4 Dwarfs
• Position: 13h:25m:12s, -11°:09′:41″

Guideposts reminder

Each month you’re encouraged to learn the new “guidepost” stars rising in the east about an hour after sunset. One reason for doing this is so you can then see how they move in the following months. If you have been reading these posts for several months, you may want to relate Spica to two earlier guidepost stars with which it forms a right triangle, Arcturus and Regulus. Here’s what that triangle looks like.

Click image for larger view. (Created, with modifications, from Starry Nights Pro screen shot.)

Click here to download a printer-friendly version of this chart.

Once you have identified the Right Triangle, note carefully the positions of Spica and Regulus. They pretty much mark the “ecliptic.” This is the path followed by the Sun. Also, within about 9 degrees north or south of it, you will find the planets and the Moon. That’s well illustrated in 2012 by the presence of both Saturn and Mars, very near the ecliptic, as noted on our chart.

Arcturus and Regulus are not the only guidepost stars and asterisms in the May sky. Again, if you have been reading these posts for several months, be sure to find the stars and asterisms you found in earlier months. Early on a May evening these will include, from east to west, the following: Arcturus, Spica, Saturn, Leo’s Rump (triangle), The Sickle,  Mars, Regulus, the Beehive, Procyon, Sirius, Pollux, Castor, and in the northwest near the horizon, Capella, and the Kite. Venus will be a bright evening “star” in the west, and if you look early in the month you may catch a glimpse of Sirius and Betelgeuse before they set.

Jupiter’s back-and-forth wanderings

On October 1, 2009 a nearly full moon joins Jupiter, Uranus, and Neptune in the southeast as shown here about an hour after sunset as seen from latitude 42 degrees north and longitude 71 degrees west. Chart from StrayyN oghts Pro software. Click for larger image.  .

On October 1, 2009 a nearly full moon joins Jupiter, Uranus, and Neptune in the southeast as shown here about an hour after sunset. (Jupiter is made large to indicate its relative brightness - ut it will look like a very bright star - not a small moon!) This is how the sky appears from latitude 42 degrees north and longitude 71 degrees west. Chart from Starry Nights Pro software. Click for larger image.

The idea here is simple – connect what we can see in the sky this month with what’s actually going on. We’ll do this by watching Jupiter, the easiest object to find right now since it is the brightest “star” fairly high in the southeast shortly after sunset.

With just a few quick checks with binoculars we should be able to track the movement of Jupiter in relation to a bright, nearby star. You should start this project on or before October 1, 2009 if at all possible and plan to observe two or more nights between your start time and October 13. Then observe again in about a week and again near the end of the month.Your first couple of checks should show Jupiter in “retrograde” moving westward among the background stars. Your next two checks should show Juputer has resumed it’s normal eastward movement.

Use the following chart as both your guide and your log. That is, click on it to get a version you can print, take out under the stars, and record your observations on with a pencil.

Click for larger version, suitable for printing.

Click for larger version, suitable for printing.

So why does Jupiter appear to first go one way, then the other? Afterall, it isn’t really doing that, is it? Like the other planets – and us – it’s simply continuing a steady, eastward journey around the Sun. But so are we – and we are moving much faster because we’re much closer to the Sun. So what you are seeing is partly the movement of Jupiter – but also the apparent change in its position caused by our rapidly changing position.

I made the following animation from Solar System Live charts. It shows how Jupiter’s position changes slowly in relation to Earth and the other planets, particularly Neptune. The animation starts with September 1, 2009  and moves a month at a time for six months. The arrow shows our changing view of Jupiter with relation to Neptune, a much more distant – and even more slowly moving, planet. Notice that in late December Jupiter makes another close approach to Neptune – the third this year – which will make especially easy at that time to find this distant and faint planet. Right now you can use the chart above to track it down – it would be just visible in binoculars on a moonless night.

picasion.com_8320c15f05e4065bb6a5159017c4c205

So let’s review the movements we’re dealing with here.

1. The daily rotation of the Earth causes Jupiter to appear to rise inthe east and move westward as the night progresses.

2. The revolution of the Eartha round the sun at a much higher speed than Jupiter makes it so that for some time the huge planet appears to be moving westward in relation to background stars and the more distant planet Jupiter. That apparent westward motion comes to a stop October 13, 2009.

3. Jupiter’s own motion is more apparent after October 13, as it appears to move eastward against the background stars. This general motion will carry it about 30 degrees eastward – very close to where Uranus can be found now – in about a year. It takes Jupiter almost 12 of our years to make a complete circuit of the sky.

The incredible variety of stars – and how much we can learn from simply noticing their color!

Even a candle flame has a lot to say when you dissect it's light. Think of what the stars can tell us!

Even a candle flame has a lot to say when you dissect it's light. Think of what the stars can tell us!

Look at a star. Can you tell it’s color? Don’t be surprised if you can’t. At first all stars just seem white to many people.  Yet amateur astronomers joyfully describe them in exciting hues of red, orange, yellow, and blue.  The truth is, it would probably be more accurate to talk of these “colors” as “taints.”  A red star doesn’t look like a red Christmas tree light. But once you’ve taken a good close look – once you’ve made comparisons between key bright stars, such as Spica and Antares in the summer sky, or Rigel and Betelgeuse in the winter sky – well, you definitely should see that Antares and Betelgeuse are tainted red and Spica and Rigel are tainted an icy blue.

This drawing - details and copyright can be found here - does a better job than anything I've seen of depicting star colors as we see them with our naked eye. Keep this in mind as you learn more about the OBAFGKM classification system.

This drawing does a better job than anything I've seen of depicting star colors as we see them with our naked eye. Keep this in mind as you learn more about the OBAFGKM classification system. For more details and copyright information click image.

And from that single piece of information – the color –  you can make reasonable guesses about a star’s size, temperature, intrinsic brightness, life expectancy – even how rare it is and the way it is going to die. How? By knowing the secret of a wonderful little classification system that uses the sequence of letters OBAFGKM – a system built on more than 150 years of breaking star light into its constituent colors, studying the result, and comparing those results with what we can learn in the laboratory about light and its relationships to various elements.  Oh – and if you don’t mind risking a little political incorrectness, you can remember that sequence of letters by this wonderful little  mnemonic device – Oh Be A Fine Girl (Guy) Kiss Me.

And what all of this tells us about stargazing in general is that what we see when we look at the stars on a typical night are not the ordinary stars, but the extraordinary ones.  That is,  the stars we see with our naked eyes are a sampling of the unusually bright, unusually close – and in some cases – unusually large . With our naked eye we cannot see a single example of the most common type of star – not one.

From a BB shot to a mountain

Color and brightness diffeences can be obvious, but the real differences are only revealed through careful study with sophisticated instruments.

Color and brightness diffeences can be obvious, but the real differences are only revealed through careful study with sophisticated instruments.

And while we certainly notice the differences in brightness of the stars, and we may have now tuned ourselves to detect the subtle tints of color that separate red stars from yellow and orange from blue, there’s little we can discern with our naked eye that prepares us for the incredible variety of stars. Take one measure alone – spatial dimensions. All stars appear to us as a point source of light – they show no disk, except as an artifact of our telescopes. In short – they all look the same size – very, very small.

Yet this sameness is a far cry from reality. In reality stars have an incredible range of sizes. Let’s scale things down to the graspable. We’ll reduce our home planet to the size of a small bead, 2mm in diameter – less than one tenth of an inch. We’ll let that small bead represent the size of a white dwarf star. On that same scale, our star – the Sun – is about 9 inches in diameter. And on that scale the largest stars we know would be almost 2,000 feet in diameter.  Think of it! Stars range from something smaller than a BB shot to something larger than the tallest man-made structure. Then consider this – that BB shot represents something the size of the Earth, a typical size for a white dwarf star – but there’s actually a type of star much smaller than a white dwarf.  It’s called a “neutron star,” and while it may be as massive as our Sun, it’s about as big as a large city! I couldn’t figure out how to include that in our little model without using a powerful microscope.

(Magic interlude: to really get a quick handle on the size of planets and stars, go here (link opens in a new window) – then return 😉

This incredible size range alone should tell you that stars are quite different, one from the other. We first began to notice how different about 150 years ago when scientists combined a prism with a telescope and broke down the light from the stars into its various wavelengths, displaying a spectrum. In such spectra we see not only colors but thousands of dark lines at locations where specific wavelengths of light are absent, and these dark lines hold the key. From them, scientists can deduce chemical composition, temperature, and much more.

The Harvard connection

In the late 19th Century astronomers at Harvard University started to classify the spectrum of stars. There were obvious vast differences, and they applied an alphabetical system to rank stars by the strength of the lines that represented hydrogen.  At first it all made sense and the letters were in alphabetical order, but as their knowledge grew, the letters got scrambled until we finally ended up with a system that runs in this order:

O B A F G K M

So Astronomy students, in an age when we were less tuned in to sexism, learned the simple mnemonic Oh Be A Fine Girl Kiss Me.  More recently they have messed things up a bit with additional letters, L and T, and within each class there are nine different sub-classes, which are represented by numbers.  But for now let’s stick with the basic letters, as they cover the vast majority of stars.  Learn it anyway you like, but learn it. The order of the spectral letters is one of the few things I find worth memorizing, for it turns out to be an almost ideal classification system covering in a single order several major characteristics of the stars.

OBAFGKM

The mains pectral classes of stars with the class designation on the left, a represenative spectrum for that class of star, and then the temperature for that spectral class on the right in degrees Kelvin.Clcik on image for information about its source, copyright, etc.

First, these letters represent a temperature sequence. The hottest stars are at the top. and classified as “O” stars. “M” stars are the coolest common stars.

And OBAFGKM represents a mass sequence, with the stars at the top being the most massive.  (Don’t get mass and spatial dimensions confused, however.  A star can have the same mass, yet be as tiny as the Earth, or far, far larger than our Sun. )

But these same letters also represent a color sequence going from blue-white to red, with yellow stars in the middle. Our Sun, for example, is a “G” star – basic yellow.

The sequence also indicates something about life span. The hotter a star is, the shorter its life. “O” stars will live a few million years, furiously exhausting their nuclear fuel. “G” stars such as our Sun, are destined to live billions of years – about 10 billion for the Sun. And “M” stars may go on for trillions – much longer than the universe has existed to date.

And the sequence tells you something about how the stars will die – those near the top, the “O” stars, will go out with a bang, those near the bottom, a whimper.  So temperature, mass, color, life span, and the end game are all related.

They also represent a sort of frequency distribution. “O” stars are one in a million – very rare.  And “M” stars are most common. In fact, about 80 percent of the stellar population is believed to consist of “M” stars, yet we don’t see a single normal “M” star with our naked eye.  (Catch the hedging there?  We do see some abnormal “M” stars. More on that later.)

Know the spectral type, know the star

But the OBAFGKM sequence really hits the jackpot when it comes to dropping stars into convenient little boxes.  If you look up at Vega and note that it is blue/white, then you know it’s nearer the upper end of the list – probably a “B” or “A.” This means it is hotter than the Sun, more massive, and will live a much shorter time. That’s a lot to know just from a crude estimate of its color, but it’s pretty close to what astronomers have learned with more sophisticated techniques. Vega is classified as an “A” star which  has a temperature of 9600K compared to the 5777K of our Sun.  It has half again as much mass as the Sun and is expected to live less than a billion years, compared to the 10 billion for our Sun. In short, know the color – or better yet, the letter assigned to a star and you immediately know something about its color, temperature, mass, life expectancy,  and death,  as well as how common it is or isn’t.

But this really intersting thing come when you make a very simple graph that ranks stars by just two qualities – their actual luminosity  and their temperature/color. “Luminosity” means the actual brightness of a star independent of its distance from us. Color and temperature are indicators of the same thing. The result is what’s known as a Hertzprung-Russell diagram. Here’s an example with each dot representing a star.

526px-HRDiagram

What you should notice is the bulk of stars fall into three areas of the charts – the lower left quadrant where oyu find the white dqaefs, the upper right region where you find various giants, and a thick, swirling band that cuts across from one corner to the other. This band hold the most stars by far and it is known as the “main sequence.”

One thing I don’t like about the word “sequence,” however, is people assume that to be “on the main sequence” means a star is sort of sliding down it – that the sequence is a developmental or evolutionary sequence.  In fact, that was what scientists believed at first – that stars started out hot (top left), then got cooler as they aged, eventually ending up in th ebottom right.  Sounds logical. But the main sequence is not a developmental sequence.  Stars don’t start out life at one end and end up at the other. Instead they stay on the main sequence until their final stages of life, at which time they may move off the main sequence into another realm entirely.  Such moves signal dramatic changes in the star’s life, size, behavior, appearance, and life expectancy.

But start witht he idea that when a star starts life it does so at some particular point ont he mains equence and it stays at that point for most of its life. Where stars go as they leave the main sequence and exactly what happens to themt hen is a matter of mass. In fact, mass is probably the single most important characteristic of a star. You would think it would be chemical make up, but notice we haven’t talked much about star chemistry. That’s because for most stars its basically the same – 92 percent hydrogen, 8 percent helium, and everything else crammed into a fraction of one percent, if it’s there at all.  In a few cases, however, chemistry is significantly different and those stars get their own special classifications. Again, we’ll leave the exceptions to another time.

To appreciate the importance of mass you need to understand what a star is. Reaching for a definition that will cover the whole range of stars, James B. Kaler calls stars “self-luminous condensates of the fragmented dusty gases that fill interstellar space.” OK. As you get near the low mass end, stars do get a bit freaky and it’s difficult to fit them into the general picture. But let’s stick with the general picture where it’s easiest to think of a star as a huge ball of gas, expanding under the pressure of the nuclear furnace at its core, and held in together by the opposing inward pressure of its own gravity.

And that’s the key – gravity. There’s another word we could explore forever without getting an ultimate answer.  Gravity is the force that causes stuff to be attracted to other stuff resulting in more stuff. Honest. The interesting thing about gravity is it works on “the more the more.” That is, the more stuff you have, the more attractive the force of gravity, and so the more likely you are to have even more stuff.

In the ISM – the interstellar medium where stars are born – there’s a lot of stuff – mostly gas and dust – that is spread quite thinly. But various disturbances pass through the ISM, sort of priming the gravity pump, encouraging stuff to get together in clumps which then become bigger clumps until you finally have so much stuff in one spot that the inward pressure becomes crushing. At this point a star is born, for what happens is the hydrogen atoms start bumping into one another, and when they do this, they fuse together and form helium atoms.  At the same time, a small fraction of the matter in the hydrogen atoms gets consumed by the process – matter is turned directly into energy by that most wonderful formula E=MC square – and, of course, the amount of energy is tremendous as we found out when we built atomic bombs and later atomic power plants. A little matter goes a very long way, for we’re multiplying the converted mass by the speed of light (186,200) squared, a very big number. (Mind you, I am not a physicists. I try to recount these things as I understand them from my study. But having read a great book that traces the history and development of that equation it still leaves me completely bewildered why the energy produced should have anything to do with the speed of light. )

What’s so elegant about a star undergoing nuclear fusion is the wonderful way it counterbalances the force of gravity. Left on its own, gravity would have just kept squeezing the stuff tighter and tighter. But the very force of gravity causes the nuclear ignition and that creates the outward force to work against the inward force of gravity.

When we see a star we’re not really staring into a nuclear furnace.  The nuclear fusion in the core is a just the starting point for an exchange of energy that takes something in the order of a million years to reach the surface of the star and send out radiation in the form of the light that we see.  But oh my, what a mighty engine!  A star such as our sun “burns”  4 million tons of hydrogen a second, and according to Leon Golub and Jay Pasachoff in “Nearest Star,” the result is that in just one-thousandth of a second the Sun emits enough energy “to provide all of the world’s current energy needs for 5,000 years.”

And we look up there and it’s hard to not hear the echoes from our nursery –  “Twinkle, twinkle, little star.” Twinkle indeed!

OK – back to OBAFGKM.  How hot a star gets – and many other characteristics – depends entirely on how much stuff is gathered together in this ball of gas. The usual measure of this is to compare the amount with how much is in our Sun. Count the Sun as “one” and a typical “O” star is made of somewhere between 10 and 100 times as much stuff, while an “M” star may have just one tenth what our Sun has.

Other boxes for the stars

Without going into tremendous detail – it’s enough to try to swallow the main sequence in a single gulp – there are four other ways that stars are classified.  I want to mention them here because you’re bound to hear these terms, and some of them sound downright crazy – like calling our Sun, which is larger and brighter than most stars, a “dwarf.”

Stars fall into two broad evolutionary categories, with a third one that exists only in theory. Stars we see belong to either Population I or Population II.  Our Sun and most other stars we see are Population I. The oldest stars we see – such as the ones that are found in globular clusters – are Population II stars.  Population III stars were the first stars formed after the Big Bang. No one has yet seen a Population III star – they are a theoretical concept, but generally well accepted as such. A Population III star would be almost entirely hydrogen and helium. Population II stars were formed next – after the Population III stars had exploded and added “metals” to the universe – metals being elements other than hydrogen and helium.  So Population II stars have a more complex chemistry. Population I stars tend to be richer in metals.  They are the stars being formed today.

(Confused? Do you get the feeling astronomers count backwards?  Well stay tuned. )

Where dwarfs aren’t dwarfed by anything but giants

Another classification system looks at the size – the spatial dimension, not mass – of stars, and it would be confusing, if it weren’t so laughable. I’m not quite sure how astronomy ended up in this quandary, but I assume it’s another instance where new discoveries played havoc with the established naming process. Sort of like trying to straighten up a room as the kids are playing in it. Thus we have a size classification system that goes from smallest to largest stars and reads like this:

VII – white dwarfs
VI –  subdwarfs
V –  dwarfs – main sequence
IV –  subgiants
III –  giants
II –  bright giants
I –  supergiants, and yes
0 –  hypergiants

Oh boy! Wonder what happened to “normal?” Well it’s there. Normal is “dwarf.” In fact the main sequence holds about 95 percent of the stars, and you can consider “dwarf” and “main sequence” synonymous.  And that, of course, means our Sun is a “dwarf.”

OK – maybe I’m being too hard onthe astronomers. As mentioned earlier, the difference in the size of stars is mind-boggling. Here’s a good graphic thatc aptures only the top half od these differences – the difference between our “dwarf” Sun and the giants.

Star sizes 2

Remember, stars do not evolve along the main sequence, but they can fall  – or jump – off it. That’s where these classifications come in. Essentially, as a star nears its life’s end, it goes careening off the main sequence in what results in graceful curves when you start plotting temperature and mass on a graph.  The path to the end is complex and varies according to the initial mass of the star. In the later years of their lives, stars can swell up to incredible sizes and become red giants. An example is Betelgeuse, the star that marks the right (eastern) shoulder of Orion. Such a star doesn’t grow in mass, but it expands like a balloon and while its surface is relatively cool – thus the red color – it’s huge and so the star radiates a lot of light despite being relatively cool..

Stars actually can make the climb off the main sequence more than once and the second time they do this, they may turn into slowly pulsating giants, varying their output significantly over long periods. (Long, that is, from our viewpoint.) Such a star is Mira, “the wonderful.” It goes through an 11-month cycle where it reaches a peak making it easily visible to the naked eye, then it falls back to a point where it is so dim that’s it difficult to detect with binoculars. Chi Cygni is another such star, and there are many more.

In these end games, stars may go through a nearly explosive stage where they blow off a huge amount of their substance and create an expanding cloud that we see as a beautiful planetary nebula. The Ring Nebula in Lyra and the Dumbbell Nebula in Vulpecula are examples.

Stars can also undergo an incredible collapse where their core shrinks to the size of the Earth. Such stars still retain a significant amount of their mass, and they are known as “white dwarfs.” In this case the word “dwarf” makes sense, but the word “white” is another name that has been overtaken by new discoveries. White dwarfs can be red, or any other color normally associated with stars.

And, of course, stars can go out with a bang if they start life with enough mass. The result is a supernova that momentarily shines with incredible brightness, then leaves behind a ragged cloud such as we see with M1, the Crab Nebula.

This doesn’t mean, however, that the star was destroyed in the massive explosion.  In fact, although everyone seems to write about the life and “death” of stars – and I’ve fallen into that pattern as well – I’m not at all sure it’s appropriate. Stars don’t die. They go through incredible changes that may make them difficult or impossible for us to see – but some of the stuff of the star is still there and in most cases it still continues to radiate light. In the case of the Crab Nebula, the star that exploded left a significant core in the form of an incredibly compact neutron star – a star where all the atoms have been stripped of their electrons and protons and are crunched together so tightly that they are about the size of a large city.  Such a neutron star spins as it collapses, much as a figure skater does when she pulls in her arms. It also beams radio energy in one direction and this beam sweeps the heavens like a powerful lighthouse. When we’re aligned with such a neutron star, our radio telescopes pick up a regular pulse of energy, many times a second, from the rapidly spinning neutron star. We call this a pulsar, and again, many examples have been discovered.

In the case of our Sun the current betting is that it will “burp” a time or two creating a complex planetary nebula, then retreat to the white dwarf stage – and white dwarfs go out with a whimper, not a bang. But while theys top radiating, there is still star stuff there so I’m not sure if this really represents the detah of a star.

Whatever the evntual fate, however, it all depends on the initial mass. A star like our Sun ends up as a white dwarf. A star 2-to-3 times larger than our Sun ends up as a neutron star. And a star that starts out even bigger, ends up as a black hole.

The bigger they are, the farther they fall!

Summary

I know we’ve covered a lot of ground. There’s actually much, much more that could be said about stars.  But the basic message is simple. Most stars fit into a single classification system that tells us at a glance several major things about the star. Learning that sequence of letters – Oh Be A Fine Girl Kiss Me – goes a long way in helping you make some sense  out of those distant,  twinkling,  dots of light.

Here’s a summary of this system in table form

star_table

Click table for a larger version.

  • Ninety-five percent of all stars are on the main sequence.  Most of the stars that are not on the main sequence are white dwarfs. Roughly one percent of the stars fall into one of the giant categories.
  • Stars near the top of the main sequence are rare, as are giants of any spectral class, yet when we look at the constellations we are seeing mostly A and B main sequence stars, and a variety of giants, since these are the brighter stars.
  • Notice that stars in the last two categories,  L and T, are either barely visible to us as red stars, or not visible to our eyes at all because they shine only in the infrared. We don’t know how many of these stars there are, but they could be as common as the main sequence “M” stars.
  • The lower limit for the mass of a star is 1/80th the mass of our Sun – or about 13 times the mass of Jupiter.
  • Temperatures are for a star’s surface. The interior is much hotter. The Kelvin temperature scale is the same as the Celsius one, except it starts at absolute zero. This adds 273 degrees to the Celsius scale, a minor consideration when you look at the typical temperature of a star.
  • Age – we can date star clusters by seeing what class of star remains. “O” stars die first, then “B,” etc. No dwarf “K” or “M” star has died yet – the universe isn’t old enough.
  • Understand in all of this we’re dealing with a continuum, so the numbers are just guides.

The stars are not a WYSIWYG world.  What you see is NOT what you get. But what you get is far more exciting, interesting, and elegant than what you see. That said, don’t forget that the goal here is to see, really see – to go out in the night and let the light from these unimaginably distance fires ping your brain, and when it does, mix that experiential knowledge with your abstract knowledge in the hopes of a greater awareness. Good luck!

Simply mind-boggling: Universal Measuring Sticks and Observing Logs

Measuring an 11-foot (meters) strip. (Click image for larger version.)

Measuring an 11-foot (3.4 meters) strip. (Click image for larger version.)

While simple, this project is next to impossible to depict well in a photograph because each “measuring stick” is just a few inches wide and more than 10-feet long. But build one and I bet you’ll find it a mind-bending experience!

I call this project the  Universal Measuring Sticks and Observing Logs and together these “measuring sticks” serve a simple function – they put into perspective the distance of each object you observe. And even if you don’t observe, they’ll help you get a handle on the incredible distances to the planets, stars, and other galaxies.

To do this our basic measuring unit will be the speed of light – 186,200 miles a second (300,000  kilometers a second). That, according to Einstein, is the speed limit for the universe. Nothing can go faster. So we simply ask ourselves how far will light go in a minute? An hour? A year?  Just starting with the distance travelled in a second boggles the mind – the distance that light travels in a single second would take it all the way around the Earth more than 7 times.  That is, it’s 24,902 miles around the Earth at the equator and if you divide that into 186,200 you get (rounded) 7.5 ( 40,076 km divided into 300,000 kmps gives you 7.5 as well). So our most basic unit, the light second, is already far larger than anything most of us have experienced. But at least it gives us a starting point to begin to get even more mind-numbing distances into perspective.

Materials needed for this project:
(2) 11-foot (or 3.3meter) lengths of adding machine tape*
ruler
pen or pencil(s)  – different colors helpful, but not necessary
calculator (helpful) or scrap paper

*You  can use any 11-foot strip of paper you have or create – but I found adding machine tape the easiest way to do this and it’s commonly available. Some might want to use four lengths and not use each side – or you might want to use a dozen sheets of ordinary paper. The goal is to make four scales each 10 feet (120-inches) long with little extra paper on each end to keep it neat.

We will actually make four measuring sticks, each a bit over 10 feet ( 3 meters) long. We need four because it is impossible to fit everything on a single scale and still have it readable.

Well, not absolutely impossible.  For example, the first and smallest scale used is for the solar system. On that scale, one inch  equals two light minutes. (If you’re using the metric system, then start with a scale of 25mm equals two light minutes – very nearly the same. ) That scale puts the moon just 1/100th  of an inch (a quarter of a millimeter) from Earth at one end with Neptune (and Pluto) near the other end.  But if we were to include the nearest star we observe in our northern hemisphere skies other than the Sun, it would  require a piece of adding machine tape 36 miles (57.9 kilometers) long! Possible – hardly practical. Oh – and were we to include the nearest galaxy we observe, the Andromeda Galaxy, we would need about 10 million miles (16 million kilometers)  of tape. Sort of defeats the purpose of a scale model! So it is for very practical reasons that we have created four measuring scales.

To use each scale start by putting a vertical line across your tape about 6-inches from the left hand edge. This is your starting point, which is, in all cases, Earth. To the left of this, put down the name of your measuring stick and the scale being used for that stick. To the right of this, calculate the distance to each item you observe, mark it and identify it on the scale. If you like, include the date observed.The result should look something like this measuring stick for the solar system – keep in mind this is just the first part – the whole “stick” goes onf or 10 feet.

This is the starting end of a solar system measuring stick.

This is the starting end of a solar system measuring stick.

General notes that apply to each stick

  • Light travels at 186,282 miles (300,000 km) a second. We use the speed of light as our measuring unit.
  • On the second and third scales in particular you may find that several objects are at, or near, the same distance, so to mark and identify them you will need to use the full width of the paper.
  • Distances up to 1,000 light years are pretty well known now and reasonably accurate because of measurements taken by the Hipparcos* satellite. Distances beyond this get increasingly fuzzy with many different indirect methods to determine them. For this reason you should regard all these distances as reasoned approximations. For close objects (within 1,000 light years) use sources written after the Hipparcos* measurements which were published in 1997.
  • Our distances are also an indicator of time – each distance tells us how long ago the light we see left an object. You may find it fun to mark all but the first scale with historical, evolutionary, and geological events on Earth. Such time references add to your perspective.

You can look up distances and calculate them to scale any time, but the real goal here is to reinforce the observing experience, so if you are observing , I suggest you use this more as a log and  mark your scale either when planning an observing session, or when reflecting on that session after observing. That way the abstract experience of learning and calculating distances is in your mind along with the real-life experience of observing the object.

The Scales – (If you prefer to work in metric, just change “1-inch” to 25mm – it will be close enough for these purposes.)

#1 The Solar System

Scale: 1-inch = 2 light minutes (or for those who want more precision, 120 light seconds*

Minimum distance in light hours, minutes, and seconds,  from the Earth to the moon and planets are:

  • Moon: 00:00:01.2  (that is 1.2 light seconds)
  • Venus 00:02:07  ( 2 minutes, 7 seconds)
  • Mars  00:03:02
  • Mercury  00:04:18
  • Sun 00:08:19
  • Jupiter  00:32:43
  • Saturn 01:06:28  ( 1 hour, 6 minutes, 28 seconds)
  • Uranus  02:23:35
  • Pluto 03:58:07**
  • Neptune 03:59:25

To calculate the distance on your Universal Measuring Stick, simply divide the time in minutes by 2, or the total time converted to seconds, by 120.

Example: Jupiter is 32 minutes, 43 seconds away. In seconds that is (32X60) + 43 or 1,920 seconds plus 43 which is 1,963 seconds. 1963/120 = 16, so Jupiter will be 16 inches away.

*Use 2 light minutes for reasonable approximations, or get more precise with seconds.

** yes, Pluto when closest to us is closer than Neptune when closest to us!

If you’ve made the solar system measuring stick, you should have the basic idea how and find the others easy.

#2 Our Stellar Neighborhood – to 2,600 light years

Scale: 1-inch = 21.6 light years

This scale covers most of what you can see with your naked eye –  as well as many things you can not see with the naked eye because they are too faint, but still fairly close to us. Well, close as astronomical objects go, but incredibly far away when it comes to what we’re used to.

(The entire solar system scale would be so small, it would be impractical to represent it on this scale with anything except the thinnest of lines right at the start.)

We’ll use some of our bright guidepost stars just for starters. Here are their distances in light years:

  • Polaris  430
  • Arcturus  37
  • Spica 262
  • Antares 600
  • Vega  25.3
  • Altair 16.8
  • Deneb 1,400
  • Big Dipper  80*

*This is an approximation covering the main stars of the Dipper which are really part of an open cluster.  With most asterisms the stars would be at various distances.

#3 Our home galaxy, the Milky Way – to 100,000 light years

Scale: 1-inch = 833 light years

(The previous scale would take up little more than the first three inches of this scale.)

This measuring stick takes us to some of the more distant open clusters, typical globular clusters, and some nebulae that are easy to observe with the naked eye, binoculars,or small telescopes.

Examples:

  • Pleiades M45 440
  • Dumbell Nebula M27  1250
  • Orion Nebula M42  1,300
  • Ring Nebula M57 2,300
  • Open Cluster M37 4,400
  • Globular Cluster M13 25,000

#4 Our observable universe – to 100 million light years

Scale: 1-inch = 833,000 light years

(The entire previous scale would take up about the first one eighth of an inch on this one.)

While the Andromeda galaxy can be detected with the naked eye and observed with ordinary binoculars, most of what we include on this measuring stick takes us to the limit of what we usually observe with a backyard observatory that includes at least a  6-inch telescope. We can reach farther into the universe than this, but with anything past the middle point on this scale you see very little – and most of what you see at these distances justifies the term amateur astronomers usually use for these objects – faint fuzzies!

Examples:

  • Andromeda Galaxy M31  2.5 million
  • Pair of galaxies beahind the Great Bear’s ears –  M81, M82  12 million
  • Whirlpool Galaxy M51 23 million
  • Leo Triplet Galaxies M65, M66, NGC 3628 35 million

Finally, getting the measure of the universe – here’s a brief tribute to the measurers – ancient and modern . . .

“HIPPARCHUS OF NICEA must have been an interesting fellow. He was a second-century B.C. mathematician, philosopher and astronomer. Using the only astronomical instrument available to him — his eyes — Hipparchus took on the daunting task of measuring the positions of the stars and planets as they passed overhead each night. He came up with a catalog of 1,080 stars, each of which he described simply as “bright” or “small.”
“Hipparchus wasn’t the first astronomer to pursue the science of astrometry, as the astronomical discipline of positional measurement is now called. However, his star catalog was the first of many compiled over the centuries by astronomers using ever-better instruments and techniques. From those measurements — all made from the Earth’s surface — astronomers have derived everything from basic stellar properties to estimates for the age of the universe.
“On August 8, 1989, the science of astrometry took a long-awaited leap to the stars. Riding aboard an Ariane rocket was the High Precision Parallax Collecting Satellite, otherwise known as Hipparcos. For the next three and a half years, Hipparchus’s 20th-century namesake measured the parallaxes and brightnesses of more than a million stars — despite a potentially crippling accident that sorely challenged the project’s architects.”

The above is quoted from this Web site: http://tinyurl.com/yurcq2 Go there for more details.

Build an inexpensive, simple, one-tooth-pick, global, equatorial, elegant and smaller than an iPod, wristdial!

A day in the Sun – a brief timelapse video of the garden-size version of the wristdial in action. (See if you can discover what time did the bird land on the dial, casting it’s shadow on the face!)  This is the same basic design as the wrist dial, only larger.

Is the wristdial really all those things – inexpensive, simple, one-tooth-pick, global, equatorial, elegant and smaller than an iPod? Yes, and with no moving parts to break. Instead it depends on the motion of the Earth which, ponderous as it is (6.6 sextillion tons) moves like – well, like clockwork!

And simple?

Yep! Here’s an image of the final product in action in the northern hemisphere early on a summer morning.  (Oh, it can also find north for you, so it can double as a compass. More on this in the full directions in PDF format 😉

composite_main

Click for larger image.

The wristdial travels in a neat, folded package (see inset at upper left). The toothpick is inserted at a right angle to the dial face, and this can be checked with the “setting triangle.” The same triangle is then used to set the dial face so its plane points to the celestial equator. Get those two set correctly, and the dial works anywhere on Earth that the Sun is shining.  clay_winterThe dial has two faces, the one shown in the preceding picture is for use in spring and summer. The other side –  shown in the picture at right – is for use in fall and winter. The design can easily be scaled up and faces are included for a larger version, or you can use the instruction included here to design your own.

Sundials are simple things that point to profound truths about the motions of Earth and Sun. They’ll teach you about your position on this rapidly spinning sphere and put you in direct touch with some awesome forces of nature. That’s what I love about them. But right now you’re probably more interested in how to make your own wristdial, so let’s do it!

You can download the full directions – with many color photos – from the link below. This is a large Acrobat PDF file, so allow several minutes for it to download.  Because of the numerous color image, I suggest you read  it on screen – but you’ll want to print either page 4 or page 5, depending on the hemisphere in which you will use the dial.

Download complete directions for wrist dial here: directions_wristdial_f4

You might want to get a jump on things by first finding and jotting down a few useful facts.

Latitude and longitude (http://www.getlatlon.com/) – You don’t have to be super precise. All can be rounded to the nearest degree. For Westport, MA I round my latitude to 42° N, and my longitude to 71° W.

Central meridian (http://www.travel.com.hk/region/timezone.htm) – Time zones are set every 15 degrees of longitude so you’ll see the central meridian for yours at the top of the map on the web page linked above. Westport, MA, is in the Eastern Standard time zone which is centered on 75 degrees longitude.

Compass deviation (http://www.geo-orbit.org/sizepgs/magmapsp.html) – I suggest you find your compass deviation only because I’m assuming you might use a magnetic compass to find north. If you have another way to determine north, you can ignore this. But a magnetic compass is not precise. In the case of Westport, MA the deviation is 16° east, which means that if my magnetic compass says it is pointing north, it is really pointing 16 degrees to the west of north, so to point true north I have to correct by pointing 16 degrees to the east of what it says is north. Of course, I might use a GPS, or call the local airport to learn the compass deviation.

Southern hemisphere dial at work in winter.

Southern hemisphere dial at work in winter.

The wristdial has now been tested in the southern hemisphere by my friend Dom in Sydney, Australia. Dom took some photos of his wristdial in action, next to a larger, traditional garden sundial in Centennial Park.  You will note three things about these photos. First, the shadow is on the underside of the dial face because when these photos were taken in mid-July it was winter in the southern hemisphere. Second, the time indicated by the dial is almost exactly the same as the time indicated by Dom’s watch. That’s because Sydney is not on daylight savings time in the winter. Also. Sydney’s longitude is just one degree – four minutes – east of the central longitude for its time zone.   Because of that the time should be four minutes fast. But, the equation of time for July  is six minutes slow. When you apply the equation of time,  the four minutes “fast” caused by a difference in longitude is subtracted from the six minutes “slow” of the equation of time and the dials solar time is within about two minutes of standard clock time.  (This kind of calculation is described in detail  for your location in the directions you can download. )

Southern hemisphere wristdial showing solar time as compared to clock time while sitting on a traditional sundial in a park in Sydney, Australia.

Southern hemisphere wristdial showing solar time as compared to clock time while sitting on a traditional sundial in a park in Sydney, Australia.

Step 8 – Directions in the sky – sometimes east is west!

Knowing the major directions in the sky – north, east, south, and west – can be confusing, but it is easy if you remember these two rules:

The direction the stars appear to move is how we define “west.”
The direction from a star to Polaris is how we define “north.”

Notice that these are new definitions. We are not talking about the cardinal directions – north, south, east, and west – as they appear on the horizon, though these are closely related. These sky directions are a bit different because we are looking at a sphere from the inside – the dome of the sky. They are absolutely essential, however, for talking intelligently and usefully about where things are in the sky in relation to one another.

Terms such as “above” and “below” are relative and not always that helpful when trying to find your way around the sky dome. Instead, learn to think in terms of the cardinal directions, north, south, east, and west.

So face south. This puts east to your left and west to your right.

Hey that was easy! Yes it was.

Now face north. East is now to your right and west to your left.  Wow – there’s nothing to this! Nope. Nothing to it – until you look at the section of sky beneath the North Star.  Now east and west get flipped. Now – in the sky – west is to your right and east to your left!

Remember – this is the special case that applies only to objects that are below the North Star. This chart should help you understand why.

 

West is always the direction the stars appear to move as theyc ircle the north celestial pole, marked approximately by the North Star. (Click image to see a much larger version.)

 

Why the change in direction? Nothing has changed really. Remember Rule 1: The star always appear to rotate to the west.  Since they appear to circle the North Star – the North Celestial Pole really – then beneath it they will appear to move from left (east) to right (west.) It is confusing because the western point of the horizon is still to your left – but you are not dealing with the horizon – you are now dealing with the sky dome.

Now lets look at the second rule. It applies everywhere in the sky, no matter what direction you face. North is always toward the North Star. We’ll illustrate this by looking north.

North is always towards the north celestial pole, marked approximately by the North Star, Polaris.

North is always towards the north celestial pole, marked approximately by the North Star, Polaris. (Click image to see a much larger version.)

Meridian and celestial equator

Let’s examine this sky dome a bit more and draw some imaginary arcs on it. One we’ll call the meridian and the other the celestial equator. Here’s how they’ll appear to us as we look toward the southern horizon.

Facing south at latitude 42° North the celestial equator appears to cross the meridian at a point 48 degrees above the southern horizon. (The  double line marks the meridian. I'm not absolutely sure why it is double in this image, but that's how my version of the free "Stellarium" software represented it. All the image son this post come from Stellarium which can be downloaded here. (Click this image for a much larger version.)

Facing south at latitude 42° North the celestial equator appears to cross the meridian at a point 48 degrees above the southern horizon. (The double line marks the meridian. I’m not absolutely sure why it is double in this image, but that’s how my version of the free “Stellarium” software represented it. All the image son this post come from Stellarium which can be downloaded here. (Click this image for a much larger version.)

Think in  terms of two huge circles.

The meridian is an imaginary circle that runs through the north and south celestial poles. Think of it as starting at the North Pole, running up through the North Star, on over your head, and down to the point that is due south on your horizon.

The second huge circle is a little more difficult to locate precisely. It is a projection of the Earth’s equator onto the sky dome and is called the celestial equator.

The celestial equator runs from the eastern point on your horizon (due east) to the western point (due) west. But it does not cross directly over head. Instead it makes a huge arc and goes through a point on the meridian that is exactly 90 degrees south of the North Star.

Finding the celestial equator in your sky isn’t as difficult as it may seem. Let’s return to that half circle, the meridian. As half a circle, it totals 180 degrees. It starts at the northern horizon and climbs to the North Star. How many degrees is that? It depends on your latitude. Here in Westport, Massachusetts, it is approximately 42 degrees. That’s the first point on our imaginary circle – our meridian.

The next point on this circle will be the point directly overhead. We call it the zenith. That will be 90 degrees above the northern horizon – 48 degrees beyond the North Star.

Now head south from the zenith – the point directly overhead – and you will find that in an additional 42 degrees you have reached the point where the celestial equator crosses the meridian. At this point there are still 48 degrees of meridian left to take you to the south point on the horizon.  This is much easier to see in a simple diagram than it is to imagine from words. But remember this easy rule:

To find the point where the celestial equator crosses the meridian at your location, simply subtract your latitude from 90 degrees. The remainder is the height of your celestial equator above the due south point on your horizon.

Do this for Westport, MA, and you get 90 minus 42 equals 48 degrees.  So face south and use your fist as a measuring stick, estimate how high 48 degrees is. That is the point where the celestial equator crosses. To see the entire equator in your mind, draw an imaginary arc from due east through the meridian at this point to due west. That’s the celestial equator.

Do you get the concept? To test yourself, try to answer these questions without reading on.

If you were standing at the north pole – latitude 90 degrees, where would the celestial equator appear to you?
If you were standing on the equator – latitude 0 degrees – where would the celestial equator appear to you?

Think you have the answers? Ok, read them here.

At the north pole the celestial equator would appear to be a projection of your horizon running from east, through the due south point on the horizon, to west.  At the equator, the celestial equator would run from east to west, passing through your meridian directly over head – at the zenith. Confused? Return to the simple rules:

The direction the stars appear to move is how we define “west.”
The direction from a star to Polaris is how we define “north.”
The zenith is the point directly over head.
The meridian runs from due north to due south passing through the North Star and the zenith.
The celestial equator runs from due east to due west making an arc that crosses the meridian 90 degrees, minus your latitude, above the due south point on your horizon.

Exercise: Test you sense of direction!

Brilliant Vega is the brightest star in the constellation of the Lyre.  See if you can find the other stars as outlined in the image below. Or, find just the two that form a distinctive triangle with it. These may not be visible in light-polluted skies, but you should see them if you look at Vega with binoculars.

Click on this to get a larger image suitable for printing and using inthe exercise below.

Click on this to get a larger image suitable for printing and using inthe exercise below.

Your assignment is this:

1. Click on the image above and when you get to the larger version, print it.Draw arrows from Vega indicating the directions of North and of West.

2. While observing, determine the cardinal directions in relation to Vega and the associated stars, starting with North.

3. What then is the direction from Vega to Sulafat, the star in the lower right corner  – or from Vega to the star in the triangle that is below and to the right in this drawing.

Scroll down for the answer.

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Click to enlarge.

The direction from Vega to Sulafat is roughly southeast. Click to enlarge.

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